Avoid creating map literal in flutter.gradle multidex check (#113845)

Author: GaryQian

packages/flutter_tools/gradle/flutter.gradle

@@ -244,7 +244,7 @@ class FlutterPlugin implements Plugin<Project> {
             project.android {
                 defaultConfig {
                     multiDexEnabled true
-                    manifestPlaceholders = [applicationName: "io.flutter.app.FlutterMultiDexApplication"]
+                    manifestPlaceholders.applicationName = "io.flutter.app.FlutterMultiDexApplication"
                 }
                 buildTypes {
                     release {
@@ -263,7 +263,7 @@ class FlutterPlugin implements Plugin<Project> {
             project.android {
                 defaultConfig {
                     // Setting to android.app.Application is the same as omitting the attribute.
-                    manifestPlaceholders = [applicationName: baseApplicationName]
+                    manifestPlaceholders.applicationName = baseApplicationName
                 }
             }
         }